The present invention relates to an ultrasonic motor which generates driving force by exciting an elastic wave using a piezoelectric element such as a piezoelectric ceramic or the like, and more particularly relates to the material and structure of the moving body of the ultrasonic motor.
The ultrasonic motor whereby driving force is generated by exciting a flexual vibration in a vibrating body comprised of a piezoelectric element has gained wide attention in recent years.
The prior art of the ultrasonic motor is described below with reference to the accompanying figures.
FIG. 11a is a partially cut perspective view showing essential parts of a conventional ring-shaped ultrasonic motor.
In FIG. 11a, a reference numeral 3 denotes a vibrating body comprised of a ring-shaped elastic base 1 with plural projections 1a and a ring-shaped piezoelectric element 2 attached to the bottom surface of the elastic body 1. A reference numeral 6 denotes a moving body, which is comprised of a ring-shaped elastic body 4 with an abrasion resistant friction member 5 attached thereto.
In this example of a conventional ultrasonic motor, a steel or a stainless steel is usually used for the materials of the elastic body 4, and the friction member 5 is bonded thereto with an adhesive or other means.
FIG. 11b shows a schematic cross-sectional view of a conventional ring-shaped ultrasonic motor In this ultrasonic motor, the vibrating body 3, supported by a ring-shaped seat 7, and the moving body 6 are held in pressure contact by a ring-shaped disc spring 8 to form an ultrasonic motor wherein the driving force is outputted via a bearing 9 attached to the moving body 6.
The operation of the conventional ring-shaped ultrasonic motor thus comprised is described below with reference to FIG. 13, which shows that the moving body 6 and vibrating body 3 are held in pressure contact, and a progressive wave of flexural vibration is excited in the vibrating body 3.
The progressive wave of flexural vibration is generated as follows. At first, a longitudinal vibration is caused in the piezoelectric element 2 by applying two AC voltages with a predetermined phase shift to two sets of driving electrodes arranged thereon, and, since the elastic base 1 works to resist this longitudinal action, a progressive wave of flexural vibration is set up in the vibrating body 3 by the same effect as a bimetal: Any given point on the surface of the vibrating body 3 follows an elliptical motion due to the progressive wave of flexural vibration. The projections 1a enlarge the lateral displacement of this elliptical motion (See .zeta. in FIG. 13). The moving body 6, pressed in friction contact with projections 1a of the vibrating body 3, is rotationally driven due to the enlarged lateral displacement.
Motors considered important from the view point of actual use in the prior art are as follows.
At first, it is necessary to maintain good contact between the moving body 6 and vibrating body 3 in order to guarantee a stable drive, since the former is driven to rotate by a friction force exerted thereon. In order for that, contact surfaces of the moving body 6 and the vibrating body 3 must be finished to have a flatness of high precision lower than several microns. Metal materials usually, and ceramic materials sometimes are used for the materials of the moving body 6 and vibrating body 3 to yield a rigidity such that the precision of flatness regarding contact surfaces is kept unchanged under the pressed state and a high precision of finishing.
Secondly, wear problems between contact surfaces of the moving body 6 and the vibrating body 3 are also important. Since a friction force is exerted between the contact surfaces during the operation of the ultrasonic motor, it is extremely important to minimize the wear due to the friction force in order to guarantee stable operation of the ultrasonic motor. Wear of the contact surface worsens the flatness and the contact state thereof resulting in a bad efficiency of the motor and generation of extraodinary noises. Further, power generated due to the wear lowers the credibility of the ultrasonic motor and a system including the same.
The abrasion resistive friction element 5 mentioned above is used as a countermeasure to the wear.
In Japanese laid-open publication Sho 62-58887, in the name of the assignee of this application, there is proposed a friction element made of a composite material of carbon fibre and resin. While high abrasion resistive materials quite different from materials used for the elastic body 4 are used for the friction element, it is also known to form the elastic body 4 from aluminum and the friction element 5 by oxidizing the surface of the elastic body 4 into an alumilite layer. However, a friction element formed by an alumilite layer is far inferior in abrasion resistivity when compared with the friction element disclosed in the above publication and, accordingly, it is impossible to guarantee stable operation for a long period.
There has also been proposed an ultrasonic motor wherein aluminum and aluminum alloy is Used for the material of the moving body 6, and the same is formed to have a cross section of a predetermined configuration as shown in FIG. 12 (See, for example, Japanese laid-open publication Sho 63-174581). This enables the realization of a quiet motor with a high efficiency.
However, the conventional ultrasonic motor is not satisfactory in efficiency, lifetime and further manufacturing cost. As to the efficiency, only 30% has been attained in the conventional motor at the maximum. In order to increase the efficiency, it becomes necessary to increase the precision in finishing the contact surfaces of the moving body and the vibrating body. However, this contradicts the reduction of the manufacturing cost.